With dense single-nucleotide polymorphism (SNP) maps for 199 drug-related genes, we examined associations between 4190 SNPs and 38 commonly measured quantitative traits using data from 752 healthy Japanese subjects. On analysis, we observed a strong association between five SNPs within the uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1) gene and serum total bilirubin levels (minimum P-value in Mann-Whitney test=1.82 x 10(10)). UGT1A1 catalyzes the conjugation of bilirubin with glucuronic acid, thus enhancing bilirubin elimination. This enzyme is known to play an important role in the variation of serum bilirubin levels. The five SNPs, including a nonsynonymous SNP-rs4148323 (211G>A or G71R variant allele known as UGT1A1*6)-showed strong linkage disequilibrium with each other. No other genes were clearly associated with serum total bilirubin levels. Results of linear multiple regression analysis on serum total bilirubin levels followed by analysis of variance showed that at least 13% of the variance in serum total bilirubin levels could be explained by three haplotype-tagging SNPs in the UGT1A1 gene.